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THz basics

The Terahertz part of the electromagnetic spectrum spans between the optics and electronics regions (infrared and microwave wavelegnths); 1 THz has a wavelength of 300 μm, energy of 4 meV, and a wavenumber of 33 cm-1. Phenomena within the Terahertz energy range are based mostly on molecular rotation (H2O, O2, CO etc.) and on interactions with atomic lattice vibrations (phonons, spectroscopic signatures of explosives).

Fig. 1: A diagram of the electromagnetic spectrum demonstrating where the terahertz region stretches with respect to the other main areas of the spectrum.

The THz range, some simple facts:

  • It provides an interface between electronic and optical technologies: sub-millimeter and the far-infrared (FIR)
  • It is non-ionising (it does not strip electrons from their atomic nuclei – unlike x-rays!)
  • It penetrates non-conducting materials
  • In this range, the following are mostly transparent: plastics, clothes, biological substances, and paper
    (useful for security purposes).
  • The following provide good contrast for imaging: metals, water, and ceramics.
  • There is a lack of compact, reliable, and tunable sources.
  • Detectors for the THz region are
    expensive (until THz-TDS)
  • Nickname: THz Gap

Terahertz time-domain spectroscopy (THz-TDS)

THz science expanded during the 1990s with the invention of the Terahertz time domain spectrometer (THz-TDS) which is based on an ultrafast laser to excite THz sub-picosecond electromagnetic wave pulses using either photoconductivity in semiconductors (where high intensity light is used to make semi-conductors conduct) or nonlinear phenomena in optical crystals.

Fig. 2: A typical THz-time domain spectrometer schematic. A delay line is used to alter the time at which the laser pulse hits the receiver and consequently the time at which part of the THz pulse is measured. By sweeping the delay line, the THz pulse amplitude over time can be determined.

A diagram of a typical THz-TDS system is provided in Fig. 2. A femtosecond laser pulse is split into two beam paths, one becomes the receiver pulse (a measurement using the receiver is possible only when a femtosecond pulse is incident on the receiver), the other is used to excite THz at the emitter. Parabolic mirrors are used to focus and collimate the THz pulses emitted through a sample and onto the detector. The detector only conducts when it is struck by the femtosecond pulse from the detection beam line. When the detector is photoexcited, the THz pulse’s electric field causes an electric current to flow inside the detector. Using a time delay on the detector beam line to control the relative time delay between the THz pulse and when the detector can conduct, the electric field of the THz pulse can be plotted over time.

THz-TDS systems are used in industry for quality control (e.g. material coating quality detection) and research (e.g. characterising active drug ingredient crystallisation). THz-TDS is also very promising for cancer detection based on the water content of cancerous cells. Below, Fig. 3 shows the detection of a terahertz pulse being detected by a THz-TDS system along with the breakdown of frequencies present in the signal, which enables the ability to directly observe absoption lines in the THz spectrum for identification of different molecules.

Fig. 4: (top) The detection of a THz field over a hundred picoseconds. An electric field causes a current flow in a detector. (bottom) The Fourier transform of the electric field, which displays the frequencies present in the THz field plot above it.

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